The present invention relates to a jointing material made of a metal oxide and, more particularly, to a jointing material and joining method of joining a metal oxide such as sapphire with another member.
A capacitive pressure sensor chip is available which detects a pressure by detecting an electrostatic capacitance. The housing of the pressure sensor chip is constituted by a base body having a predetermined space and a diaphragm arranged on the space of the base body. The pressure sensor chip has a stationary electrode arranged on the base body and a movable electrode fixed to the diaphragm. When the diaphragm deforms upon reception of a pressure, the distance between the movable electrode and stationary electrode changes, so the electrostatic capacitance between the movable and stationary electrodes changes. The pressure received by the diaphragm can be measured on the basis of the change in electrostatic capacitance.
As such a pressure sensor chip, one in which the base body and diaphragm which constitute the housing are made of sapphire is proposed. Sapphire, i.e., corundum (aluminum oxide, α phase), is thermally very stable, hardly dissolves in an acid or alkali, and has wide applications such as in a refractory, insulator, and abrasive. When sapphire having such properties is used to form the housing, even if the measurement target is a corrosive fluid, the pressure of the fluid can be measured by receiving the fluid with the diaphragm directly.
The above pressure sensor chip is fixed to a base as shown in FIG. 5, and is used as a pressure sensor. As shown in FIG. 5, a chip 501 in the pressure sensor is placed in a recess 503 formed at the center of a base 502 made of, e.g., glass, such that its diaphragm portion faces up in FIG. 5. Terminals 505 of electrode pins 504 extending through the bottom surface of the base 502 are present in the bottom surface of the recess 503 of the base 502, and are connected to the interconnections of the respective electrodes extended to the rear surface of the base of the chip 501. A vent hole 506 communicating with the bottom surface of base 502 is formed in the bottom surface of the recess 503.
The chip 501 is pressed at the peripheral portion of its upper surface by a cover plate 507 having an opening at its center, and is fixed to the recess 503. The cover plate 507 is joined to the upper surface of the base 502 with glass which is fused once, so that it is fixed to the base 502. The abutting portions of the cover plate 507 and chip 501 are connected hermetically so the measurement target fluid which is to come into contact with the upper surface of the chip 501 does not enter the recess 503 around the chip 501.
The cover plate 507 is made of sapphire which is the same material as that of the chip 501, in order that the precision of pressure measurement is secured and that deformation of the chip 501 by the temperature becomes identical, thus causing no stress in the chip 501. The abutting surfaces of the cover plate 507 and chip 501 are joined by direct joining in order to eliminate hetero-joining, so that the hermetic state described above can be obtained. In this sapphire joining, the two joining surfaces are mirror-polished and abutted against each other. Pressure is applied across the two sapphire members to be joined, and the sapphire members are heated. Thus, a firm joining state can be obtained without using any jointing material or the like.
In above direct joining, as the abutting surfaces of the two sapphire members must be mirror-polished until their surface roughnesses become 0.3 nm or less, the member (cover plate) becomes very expensive. If a generally available jointing material or the like is used, the joining surfaces need not be mirror-polished. However, as a different material is present between the joining surfaces, a stress may occur, or the joining portions cannot have corrosion resistance or thermal stability equivalent to those of sapphire. Then, the applications are limited.
As described above, conventionally, when a component is to be formed by joining two or more members made of a metal oxide such as sapphire, to exhibit 100% the properties of the material that forms the members, the members are joined to each other by direct joining. However, direct joining is costly. In view of this, when the members are to be joined to each other by utilizing a conventionally available jointing material, a component can be formed at a low cost, but the properties of the material that forms the members cannot be utilized 100%.